The centrifugal fecal flotation test remains the best technique for detecting the most common nematode parasites in dogs and cats. Some less common nematode infections, however, are more efficiently and accurately detected with a Baermann test. This test is used when the diagnostic stage of infection is a first-stage larva rather than an egg. The Baermann test is probably the easiest morphology-based parasitologic test to perform and evaluate but still is rarely carried out in veterinary practice.

What the test detects

The most common nematode parasites detected with the Baermann test are Aelurostrongylus abstrusus in cats and Strongyloides stercoralis in dogs. Aelurostrongylus abstrusus adults are found in the bronchioles and alveolar ducts and are transmitted through ingestion of a snail intermediate host or a bird or rodent paratenic host. Infection in cats can cause signs ranging from cough to severe bronchopneumonia.

Strongyloides stercoralis is found in the small intestine and can infect dogs by several routes, including skin penetration and transmammary transmission. Strongyloides stercoralis infection may cause diarrhea, but because larvae migrate through the lungs respiratory disease may also occur.

The Baermann test is also used to detect the canine nematode parasites Crenosoma vulpis and Angiostrongylus vasorum, but these parasites have very limited distribution in North America (Table 1).

Why fecal centrifugation is less efficient

Aelurostrongylus abstrusus and S. stercoralis female worms produce eggs that hatch quickly, resulting in the passage of larvae from the host. These larvae may be found on a routine fecal flotation test, but the hyperosmotic flotation solution rapidly distorts them, making specific identification difficult. The Baermann test allows recovery of these larvae without exposing them to damaging solutions.

The Baermann test also allows a larger amount of feces to be used than in a fecal flotation test. Since larvae may be present in low numbers or intermittently, it is helpful to examine larger samples.

How the Baermann test works

In the past, the Baermann test required elaborate equipment and was never routinely used in practice. Today the only special equipment required is a disposable plastic wine glass with a hollow stem. A fecal sample is suspended in water in the bowl of the glass for a period of at least eight hours, giving larvae in the sample an opportunity to move out of the feces and into the water through random movement. The larvae are unable to swim and fall to the bottom of the hollow stem of the glass where they can be collected and examined microscopically (see "How to perform a Baermann test" on the next page).

It is imperative to use a fresh fecal sample (collected immediately after passage from the animal) in a Baermann test. When feces comes into contact with the ground it can be contaminated with free-living nematodes that will also be recovered in the test and can be hard to distinguish from parasite larvae.

In addition, hookworm eggs can embryonate and hatch in a short time in warm weather. First-stage hookworm larvae will also be found by a Baermann test and can be difficult to differentiate from Strongyloides larvae. A sample that was collected fresh but then refrigerated for a lengthy period (days) is also not recommended, since larvae may die in that time, and the test requires the presence of live larvae.